Add command line option forwarding for plugins

• include/wget/wget.h: Add API for command line option forwarding
• libwget/plugin.c: Likewise
• libwget/test_linking: Add function from libwget/plugin.c
• src/dl.c: Remove unused dl_search1() and dl_list1(), use wget_vector_t for pointer arrays
• src/wget_dl.h: Likewise
• src/plugin.c: Implement option forwarding
• src/wget_plugin.h: Likewise
• src/options.c: Add options '--plugin-opt=', '--plugin-help'
• unit-tests/test-dl.c: Use dl_list() instead of dl_list1()
• tests/Makefile.am: Add test plugins pluginoption, pluginfaulty1, pluginfaulty2
• tests/test-plugin-dummy.c: Implement additional test plugins
• tests/test-plugin.c: Extend tests for testing option processing

Command line option forwarding has been implemented as per specifications at https://gitlab.com/akash_rawal/wget2/wikis/Stage-1

Several style issues were also fixed (Thanks Ander Juaristi)